Network systems enable communication of messages among computer systems. For example, an electronic mail system (or a compatible system) enables the communication of e-mail messages between computers attached to a network. Some messages transferred over the system may not be desired by their recipient. Such messages typically are referred to as spam. Spam messages may crowd the mailbox of a recipient to such an extent that desirable messages may go unnoticed and unread by the recipient. Spam filtering applications attempt to address this problem by identifying and filtering out spam prior to message storage. Such applications, however, frequently filter out messages desired by the recipient along with the spam.